High-resolution near-infrared observations of Herbig-Haro flows - I. H2 imaging and proper motions

We present results of an H2 proper motion study of three Herbig-Haro flows: HH 7-11, 25-26 and 33/40. These are the first proper motion measurements for these objects in the near-infrared, and are complementary to a velocity-resolved, echelle spectroscopy study of the H2 line profile from these objects, presented in a companion paper (Paper II, this issue). The results presented here cover a 4-5 year time-span. The HH 7-11 outflow components have high proper motions, ranging between -200 and 450 km s-1. The directions of their propagation are remarkably uniform, leading directly away from the outflow source. This proper motion pattern is in accordance with recent numerical simulations by Völker et al., and leads us to suggest that the HH 7-11 chain is produced by a (pulsed) heavy jet, of which HH 7 is the working surface, driving into a low-density but clumpy medium. The HH 25-26 system is quite intriguing: the shock components furthest from the source have high proper motions (?200km s-1), while those closest to the source have negligibly small proper motions (?70 km s-1). This system is interpreted in terms of working surface bow shocks internal to the jet, and a turbulent mixing layer between the flow and the ambient medium. Indeed, HH 26A and 25C may represent the jet being deflected off nearby dense material. The HH 33/40 flow does not show any significant amount of proper motion, with an upper limit of ?40-70 km s-1. Situated at the terminus of the giant 'parsec-scale' HH 34 outflow, HH 33 is probably the working surface of the outflow while HH 40 must be material caught up in the general flow of the system. Finally, we show that the limb-brightened leading edges of the bow shocks in HH 7 and 33 are resolved. If the widths of these leading edges represent cooling lengths, then the shocks are probably C-type, as opposed to J-type, bow shocks driving through pre-shock material with <1-mG magnetic fields.